Since you can buy BP weapons through the mail and the internet, as a practical matter ... well, you fill in the blanks. Having said that, I agree with the idea of selling it to the parent(s) and then they can transfer or give permissive use or whatever the legal "phrase of art" is. This sounds like a responsible idea and it gets the Mom/Dad/Guardian involved in some sort of interaction with the kid (always a good thing: why do we spend more time with our stockbrokers or therapists than our children, and then complain that kids gave gone to $#*! in a handcart?) Then, the adult can take the kid to the range and they can have a good supervised time. No problems.
We certainly need as many youngsters in shooting sports, and in muzzle loading, as can possibly be arraigned. I gave each of my kids a muzzleloader on their eighteenth birthdays, made by GPD (Great and Powerful Daddy) thus ensuring a continuation of the rockbanging tradition from generation upon generation. I would encourage anyone else to do the same. All kids should have a bicycle, a dog, a cat, a goldfish, a collection of seashells or interesting rocks, a rifle built by their father (or mother, if she is better at "making things"), a pocket knife (and the ability/instruction to keep it sharp), and by the time they reach adulthood, the ability to ride a horse, get to the bottom of the hill on skis, get across the (well frozen) pond on skates, work a manual transmission, write a decent term paper, catch a fish, grow a tomato, brew a beer, write a condolence letter and balance a checkbook. Wow... my wife and I taught our kids all that?! no wonder I feel so old.
the hour is late... sorry about the tirade (and the colorful language: you need not teach you children to cuss- the schoolyard and the gunnery sergeant can do that just fine- i rec comend the gunny- the cadence and rhythms are clearer and more emotive).
go forth, and make good smoke!